Transcript Regulating Plant Growth

Unit
Plant Science
Problem Area
Managing Plant Growth
Lesson
Regulating Plant Growth
Student Learning Objectives
1. Understand plant growth regulators and
plant hormones.
 2. Identify different types of plant
hormones.
 3. Identify the uses of growth regulators
on plants.
 4. Define allelopathy.
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Terms
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Abscissic acid
Abscission
Allelopathy
Alleurone layer
Apical dominance
Auxins
Chlormequat
Creosote
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Cuticle
Cytokinins
Daminozide
Ethylene
Exudation
Gibberellins
Internode
Leaching
Terms cont.
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Plant Growth Regulators
Plant hormones
Senescence
Tannic acid
Tensile strength
Volatilization
What are plant growth regulators and plant
hormones?
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Plant growth regulators are organic
compounds, either natural, or synthetic, that
modify or control one or more specific
physiological processes with a plant.

A. Natural plant growth regulators are produced
by plants are plant hormones. Plant hormones
are naturally occurring compounds produced by
the plant to accelerate or retard the rate of
growth or maturation.
 1.
Plant hormones are produced in minute
quantities in one part of a plant and then
translocated to another part of the plant where
growth and development are modified.
 2. Plant hormones have many different effects
on plant growth and development.
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B. Scientists simulate the naturally occurring
plant hormones to artificially produce synthetic
plant growth regulators.
What are the different types and effects of
plant hormones?
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A. Auxins are produced in the apical meristem
of a plant’s stem and migrate down stem.
 1. Auxins
cause cells to elongate.
 2. Auxins are responsible for phototropic
responses or the ability of a plant to bend toward
a light source.
 3. Auxin plays a role in apical dominance.
Auxins will move down the stem and inhibit the
growth of side shoots. Pinching off the apical
meristem stops the flow of auxins and side shoots
are free to develop.
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4. Auxins attach to cell walls and activate a series of
processes that lower the tensile strength, resistance to
lengthwise stress, of the cell membrane. This allows the
cell to enlarge. Auxins, being acidic in nature, increase
the acidity in the cell membranes making the cell wall
less resistant to stretching from osmotic forces.
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5. Auxins are commonly used to promote root growth.
The faster a cutting develops roots, the better chance for
survival. Plant propagators will apply auxins to the base
of cuttings to promote root growth.
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6. Auxins delay abscission (shedding) of leaves and
maturing fruits. Auxins will work to breakdown the thin
layer of cells in the abscission zone.
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B. Gibberellins (gibberellic acid) are produced
in stem and root apical meristems, in seed
embryos, and in young leaves.
 1.
Gibberellins stimulate stem growth. They
induce stem cell elongation, cell division, and
control enzyme release.
 2. Gibberellins produced in the embryo during the
germination of cereal seeds moves to the
aleurone layer, the outer layer of the endosperm,
activating the synthesis of enzymes.
 3. Seventy different gibberellins have been
discovered to exist. A plant species will respond
to only certain types of gibberellin.
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C. Cytokinins are responsible for cell division and
differentiation. Roots supply cytokinins upward
toward the shoots.
 1. Cytokinins cause cell enlargement, tissue
differentiation, dormancy, and retardation of leaf
senescence.
 2. Cytokinins are most abundant in seeds, fruits,
and roots.
 3. The balances of auxins and cytokinins dictate
whether cells will develop shoots, roots, or remain
undifferentiated.
 4. Cytokinins slow the process of senescence, or
aging, by preventing the breakdown of chlorophyll
in leaves.
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D. Abscissic Acid is a growth inhibitor within the category
of plant growth regulators.
 1. Abscissic acid promotes dormancy in seeds and buds,
and flowering in some short-day plants.
 2. Abscissic acid prevents seeds from germinating in fruit.
 3. Abscissic acid assists in maintaining water supplies
within the plant. When a plant becomes stressed, greater
amounts of abscissic acid are produced. Abscissic acid
interferes with the availability of potassium in the guard
cells, causing the stomata to close. Increased water
supplies will breakdown the abscissic acid causing the
stomata reopen.
 4. Abscissic acid will slow or shutdown the metabolism in
plants. When abscissic acid is reduced, metabolic
processes will resume.
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E. Ethylene is a water-soluable gas that moves
readily throughout the plant. The cuticle, a thin,
waxy layer of cutin that covers the epidermis,
prevents loss of ethylene from plant tissue.
 1. Ethylene is produced in ripening fruits,
senescent flowers, plant meristems and at
sites where plant or fruit injury occurs.
 2. Ethylene is commonly used to promote fruit
ripening and flower initiation.
What are the uses of growth regulators on
plant tissues?
Each hormone promotes many different
responses, and each is effective in very
low concentrations.
 A. There are many practical applications of
auxins in the agricultural industry.
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 1. Auxins
are used in the horticultural industry
to promote rooting of cuttings.
 2. Auxins are used to thin fruit blossoms,
resulting in larger fruit, and in growing seedless grapes.
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B. There are numerous applications of gibberellins.
 1. Gibberellins play an important role in the
expression of hybrid vigor in corn plants. Larger
growing hybrids contain higher concentrations of
gibberellins than inbred plants.
 2. Gibberellins stimulate the development of flowers.
 3. Gibberellins are applied to grape vineyards
increasing the berry size by thinning clusters and
increasing internode, stem region between nodes,
length, allowing for more space for the fruit to grow.
 4. Gibberellins used on citrus crops assist in keeping
the peels tough and resistant to molds. Gibberellins
keep the peel green, and growers could use the
ripening hormone ethylene to bring fruit to its normal
color.
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C. Cytokinins have numerous applications in
today’s agricultural industry.
 1.
Cytokinins are added to media for cell division to
occur.
 2. Cut flowers that lose their source of cytokinins are
sprayed with cytokinins to extend their vase life.
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D. Ethylene has many applications for use in the
agriculture industry.
 1.
Bananas are picked green for shipment to prevent
bruising. They are treated with ethylene to promote
ripening.
 2. Cut flowers are never stored with ripening fruit or
decaying leaves that might give off ethylene and
shorten the life of the flowers.
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E. Abscissic Acid has many uses in the
agricultural industry.
 1. Numerous synthetic growth inhibitors have
been produced and used commercially in the
horticulture industry. These synthetic
regulators were developed from abscissic
acid or the way it functions from the plant.
 2. Daminozide retards growth and stimulates
flowering in some plants, including
chrysanthemums, bedding plants and
azaleas.
 3. Chlormequat (Cyclocel, CCC) is used to
reduce the height of poinsettias and prevent
lodging in wheat.
What is allelopathy?
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Allelopathy is the release of chemicals by
certain plants that inhibit the growth of
competing plants.
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A. Plants release chemicals into the soil, water
or air that have specific effects on neighboring
plants (i.e. inhibiting growth or germination). Two
examples of familiar chemicals that may be used
for allelopathy by plants include tannic acid and
creosote.
 1.
Tannic acid is found in the leaves and bark
of oak and sumac trees and is used to tan
leather.
 2. Creosote is the black tar-like substance
put on telephone poles, railroad crossties and
pier pilings to slow decay and rot. Creosote
bushes are the dominant plant in the deserts
of Texas. Scientists around the world are
studying allelopathy as a natural way to inhibit
weeds in crop fields and reduce chemical
herbicide use.
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B. There are several ways in which an allelopathic plant
can release its protective chemicals:
 1. Volatilization: Allelopathic trees release a chemical in
the form of a gas through small openings in their leaves.
Other plants absorb the toxic chemical and die.
 2. Leaching: All plants lose leaves. Some plants store
protective chemicals in the leaves they drop. When the
leaves fall to the ground, they decompose. As this
happens, the leaves give off chemicals that protect the
plant.
 3. Exudation: Some plants release defensive chemicals
into the soil through their roots. Those chemicals are
absorbed by the roots of other trees near the
allelopathic one. As a result, the non-allelopathic tree is
damaged.
Review/Summary
What are plant growth regulators and plant
hormones?
 What are the different types and effects of
plant hormones?
 What are the uses of growth regulators on
plant tissues?
 What is allelopathy?
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